It is often desired to control the thermal effect of incident radiative energy upon a given surface. For example, in the field of space satellites, it is often desirable, if not necessary, to control the absorption of radiant energy from the sun, earth, or other energy sources by thermally sensitive equipment disposed within the satellite. It may also be desirable to have given surfaces of the satellite absorb incident radiant energy at times and reflect this radiant energy at other times. Earth-based applications can be envisioned where temperature control is desired for surfaces exposed to radiant energy from the sun, blast furnaces, open flames, nuclear sources, etc.
Prior attempts at solving the problem of the thermal effects of incident visible and near-visible radiant energy (also referred to herein as "light") are exemplified by U.S. Pat. No. 3,341,274 issued Sept. 12, 1967 to Marks. In Marks, cells comprising dipole particles carried in a fluid suspension are capable of permitting the passage or reflection of light as a result of the alignment of the dipole particles through the imposition of an electrical field or disorientation of the dipole particles by the removal of the electrical field. If desired, light may pass through the suspension of dipole particles and strike a light absorbing body.
A serious problem with the use of the cells of Marks is the agglomeration of the particles. Particle agglomeration tends to remain more or less permanently even though the electrical field is removed. Such agglomeration considerably impairs the usefulness of the cells for various reasons.
A solution to particle agglomeration has been set forth in U.S. Pat. No. 3,655,267 issued Apr. 11, 1972 to Forlini. In Forlini, agglomeration for long periods of time may be precluded by using a radio-frequency energizing electrical field. Unfortunately, high radio frequencies (e.g., greater than 325 Kilohertz) would be inappropriate under certain circumstances thereby reducing the usefulness of the Forlini cells.